Distributed fiber-optic sensing uses properties of optical fibers to make measurements of, e.g., spatial and temporal behavior of a measurand field. Optical fibers are flexible, dielectric, passive, non-intrusive, and easy to install into existing structures. Distributed temperature sensing (DTS) technologies have been deployed in industries such as oil and gas production, power cable monitoring, leakage detection at dikes and dams, and integrity of liquid natural gas carriers. Temperatures are recorded, not at points, but as a continuous profile along the length of the optical fibers functioning as linear sensors, based on optical phonon interaction with relatively large frequency shifts (˜13 THz) called Raman scattering and acoustic phonon interaction relatively small frequency shifts (˜11 GHz) called Brillouin scattering.
Nevertheless, these two fully distributed fiber-optic scattering sensing schemes are limited in terms of what physical quantities they can measure. In addition to temperature sensing, strain monitoring also plays a significant role in operational safety for a variety of applications such as well integrity monitoring and downhole seismic acquisition. Strain, however, is difficult to measure, as it may not be constant across the length of the liber and may change with time before or after installation. In addition, strain may be related to temperature changes. While fiber Bragg gratings can be used as direct sensing elements for strain and temperature, they are point sensors and can only make measurements at a finite number of points along a line.
Existing Raman-based DTS systems use light intensity measurements of Raman scattered light to provide temperature determinations, which are not dependent on the strain condition of the fiber. As such, the Raman signal is sensitive to temperature only. Brillouin sensing, meanwhile, is receptive to both temperature and strain. Thus, because strain cannot be measured with Raman scattering alone, distributed strain-and-temperature sensors in a single-mode fiber use Brillouin scattering measurements alone or use a hybrid of Raman and Brillouin scattering detection in association with optical time domain reflectometry (TDR). In systems based on spontaneous Brillouin scattering, strain and temperature changes in the same single-mode sensing fiber can be distinguished by simultaneous detection of both spontaneous Brillouin intensity and Brillouin frequency shift. However, its performance is significantly limited by the poor accuracy of the intensity measurement. Other approaches use large effect area fiber to achieve simultaneous temperature and strain sensing, which creates multiple Brillouin frequency shifts within a single fiber core. This approach however leads to poor spatial resolution, limited sensing accuracy, and short sensing distance due to large interference between different wavelengths.
In hybrid schemes, the temperature profile along the sensing fiber is directly obtained by measuring a spontaneous anti-Stokes Raman signal, which is strain independent. With knowledge of the temperature of the fiber, one can then compute the strain from the Brillouin frequency shift information. This approach is limited, however, because the backscatter power levels between Raman and Brillouin are significantly different. For example, spontaneous Raman scattering measurement uses pump levels of, e.g., about 1W, while spontaneous Brillouin scattering needs narrow-band optical sources with a typical output power of below about 60 mW.
High input powers close to 1W can cause strong non-linear effects, such as modulation instability, on the Brillouin signal. If the input power goes down to 50 mW, which may occur due to fiber degradation and differential loss, then Brillouin-based DTS may continue to function, but without the Raman scattering signal there would be no way for that system to differentiate between temperature and strain effects. As such, the power requirements imposed by the Brillouin measurements do not seem compatible with the high-power optical sources needed for Raman measurements, such that measurements are made in alternation. In addition, direct detection is generally used to spatially resolve the Raman anti-stokes intensity, while coherent detection is used to spatially resolve Brillouin frequency shifts. Thus, hybrid Raman-Brillouin sensing in a single-mode fiber is analogous to monitoring two physical measurands using two distinct fiber-optic sensing systems, resulting in slow operation and high installation costs.